The purpose of a sinking fund is to enable the site to renew capital assets when they eventually wear out - rather than the whole cost to be built into one year's maintenance charge. For example, servicing a boiler is part of maintenance, but no matter how good the servicing regime, in the end a boiler has to be replaced as a capital renewal.
It is reasonably common for a proportion of an annual MAINTENANCE charge to be paid into the sinking fund each year (and held by the site management company) but clearly your site did not make this a requirement.
The other way to think about it is that she has had the benefit of the use of the assets for 22 years yet never had to renew anything yet, and other residents who have already sold up have presumably had to pay this - so there is nothing inherently unfair about it.
Unsurprisingly there is no 'law' about this - it is a contractual matter and unless you could show that the contract is unfair then it is just something that has existed in the contract since 1989.